Heretofore, adhesive tapes are stuck to surfaces for temporary fixation in predetermined positions of parts to be bonded together or parts to be bonded to housings or parts to be bonded to substrates in assembly processes, or for protection of surfaces that are readily scratched during working or conveyance. However, when adhesive tapes are used for such temporary fixation or surface protection, there are some problems in that peeling them is often difficult and, when they are peeled, the objects may be scratched or paste may remain on the objects.
Recently, vehicles, electric appliances for household use and building materials have become recycled, and structures bonded with adhesive are also desired to be recycled. In general, resin adhesives may readily melt or decompose when heated at 400° C. or higher, and the structures bonded with them may be thereby readily dismantled. However, when heated at such high temperatures, the structures themselves would be deteriorated and denatured, and it causes a problem in that the dismantled structures could not be recycled.
A transfer sheet with an adhesive sheet is utilized as an efficient transfer method, since multiple parts disposed on the sheet may be transferred all at a time. In addition, the adhesive sheet acts also as a reinforcing material for the parts to be transferred, and, for example, it enables transfer of even brittle parts such as ultra-thin semiconductor wafers and semiconductor chips with no damage thereto. In this, the transfer sheet must have a high adhesive force to objects before they are transferred, and must readily release them when they are transferred.
To that effect, adhesive sheets that have a high adhesive force for the necessary period of time but, on the other hand, may be readily peeled from objects when the objects are to be released from them are needed in many applications.
As the adhesive sheets of the type, those capable of reducing their adhesive force when some stimulation is applied to the adhesive matter thereof are known, and they include, for example, heat-foaming adhesive sheets, thermosetting adhesive sheets, photocurable adhesive sheets. However, even when these adhesive sheets are used, their adhesive force that may be reduced is limited, and, in fact, some adhesive sheets could not have a sufficiently high adhesive force before transfer, or some others could not sufficiently reduce their adhesive force in transfer.
Patent Reference 1 discloses a heat-decaying adhesive sheet that may decay by at least 95% of the sheet weight when heated at 700° C. or lower for 30 minutes or shorter. When the heat-decaying adhesive sheet is used, then the sheet itself may be removed when heated. However, Patent Reference 1 says that the heat-decaying adhesive sheet is resistant to explosion-proofing treatment of Brawn tubes, and does not decay even when heated at 450 to 550° C. for a short period of time. From this, it is considered that the sheet may take ten and a few minutes before it decays at a temperature of 450° C. or lower. It is considered that the long-term heating at such high temperatures may cause deterioration and denaturation of objects.
Further, Patent Reference 2 discloses a binder for ceramic shaping, which comprises a water-soluble polyalkylene glycol, a novel combed hydrophobic diol and a polyisocyanate, and which is such that its heat generation in degreasing is slow, the amount of its heat generation is small and the degree of carbon residue from it is small. However, the binder resin disclosed in Patent Reference 2 is an urethane resin obtained through reaction of diol and isocyanate, and, therefore, its decomposition residue may be small but it is problematic in that it leaves an yellowed residue that may be an oxide derived from the urethane bond. Accordingly, clean surfaces are often difficult to obtain in some cases.
Heretofore, various porous films are used for forming interlayer insulating films in semiconductor devices and multi-level wiring boards. For shortening the signal transmission delay time, it is desired that the interlayer insulating films of the type are formed of an insulating material having a low dielectric constant. To satisfy the requirement, Patent Reference 3 to be mentioned below discloses a method for producing a porous film by thermally processing a film of a uniformly-dissolved composite material that comprises a partially-hydrolyzed condensate of an alkoxysilane and a fluororesin having a carboxylic acid group in the molecule and having a fluorine-containing aliphatic cyclic structure or a fluorine-containing aliphatic structure in the backbone chain thereof, at a temperature not lower than the thermal decomposition-initiating temperature of the fluororesin.
However, according to the production method described in Patent Reference 3, a large number of pores are formed through the thermal treatment effected at a temperature not lower than the thermal decomposition-initiating temperature of the above-mentioned specific fluororesin, and the porous film is thereby produced. Accordingly, an SiO2-based porous material having such a large number of holes formed therein is obtained, and it is said that the film obtained has a low dielectric constant.
When the size of the pores in a porous film is enlarged in some degree, then the dielectric constant of the film may be further lowered. However, according to the method described in Patent Reference 3, it is difficult to form uniform pores. In addition, there is still another problem in that, when the fluororesin is thermally decomposed, it gives a residue, and therefore the dielectric constant of the film could not be fully reduced.
In addition, since the film has a large number of pores, its mechanical strength is significantly problematic.
Further, regarding the formation of interlayer insulating films in semiconductor devices, they are formed after the formation of elements on semiconductor substrates, and therefore, when the semiconductor substrates with elements thereon are exposed to high temperatures for a long period of time, then the elements may be deteriorated. Accordingly, it is desired to develop interlayer insulating films capable of being formed at low temperatures.
Recent ICs and LSIs are to have much more increased degree of integration and are to be much more micropatterned, and more than these, high-density packaging techniques are greatly developing. In addition, electronic system integration techniques are much desired, which are for integrating and systemizing various functional blocks such as optical devices and high-frequency devices that comprise multiple LSIs and compound semiconductors, for example, for system-in-packages.
Heretofore, for connecting semiconductor chips of IC or LSI on an electronic circuit boards, employed is a method of covering the semiconductor chip with a resin package and soldering the tip of the lead that runs from the resin package, on a printed board. In this method, however, all leads must be soldered one by one and the production efficiency is not good. In addition, the lead runs out from the side of the resin package, and the method is therefore unsuitable to high-density packaging. To solve these problems, a technique of BGA (ball grid array) has been developed, which comprises bonding semiconductor chips to a wiring board by conductive fine particles such as solder balls or bumps. According to this technique, chips are bonded to a substrate while the conductive fine particles bonded to the chips or the electrodes of the substrate are melted at high temperatures, and therefore electronic circuits may be constructed while satisfying both increased producibility and increased bonding reliability.
For bonding such conductive fine particles to electrodes, heretofore employed is a method of making multiple conductive fine particles sucked by a suction pad having multiple vacuum suction nozzles, and putting the conductive fine particles on multiple electrodes all at a time, and bonding them to the electrodes. In this case, in general, the electrode face of the electronic parts is previously coated with an organic acid ester. The organic acid ester is effective for removing the spontaneous oxide film formed on the electrode surface and for facilitating the capture of the conductive fine particles. However, the conductive fine particles thus put on the electrode are still problematic in that they may shift from the electrode while conveyed to the next step.
In the method of disposing conductive fine particles by the use of vacuum suction nozzles, the conductive fine particles must be sucked by all the vacuum suction nozzles, but suction failures may occur and the method is not always a sure method. In addition, since the conductive fine particles are only sucked, they may drop off if the operation speed is increased. Further, since the electrode position for electronic parts differs for every electronic part, and the method has still another problem in that vacuum suction nozzles corresponding to all electronic parts must be developed.
Different from this, another method has been proposed, which comprises disposing conductive fine particles on electrodes by the use of a conductive particles transfer sheet that has conductive fine particles previously disposed on an adhesive sheet. Concretely, the method comprises applying a conductive particles transfer sheet to electronic parts in such a manner that the position of the conductive fine particles corresponds to the position of the electrode, then melt-bonding the conductive fine particles to the electrode, and thereafter peeling the adhesive sheet. In this case, when the adhesive used in the adhesive sheet is a specific one capable of reducing its adhesive force by imparting specific energy thereto, then the adhesive sheet may be readily peeled off not breaking the connection of the conductive fine particles once melt-bonded to the electrodes. For the adhesive sheet of the type, for example, a heat-foaming adhesive sheet, a thermosetting adhesive sheet and a photocurable adhesive sheet have been proposed.
In fact, however, even when such an adhesive sheet is used, its adhesive force that may be reduced is limited, and the connection of some conductive fine particles may be broken when the adhesive sheet is peeled off, or an adhesive paste may remain on the surface of the conductive fine particles. In particular, when the peeling speed is increased so as to increase the working efficiency, then this tendency becomes remarkable. On the other hand, if the adhesive force of the adhesive sheet is made low, then this causes another problem in that the conductive fine particles may drop off.
On the other hand, there is proposed still another method of using an adhesive sheet with conductive fine particles embedded in the position opposite to the electrodes to be bonded, and disposing the conductive fine particles on the surface of the electrodes and bonding them (for example, see Patent Reference 4). However, when kept under severe conditions, for example, when kept in high-temperature high-humidity environments or dipped in solvent, the resin to constitute the adhesive sheet may be deteriorated or deformed and, as a result, the embedded electroconductive fine particles may receive some stress that acts to separate the particles from the electrode face. Accordingly, the method of not using the adhesive sheet is better in the current situation for ensuring high bonding reliability.
In such high-density packaging techniques, the most important elementary technique is a fine bonding technique. Heretofore, one typical bonding technique for it is a bump-bonding technique. With the development of the above-mentioned packaging technique, further increase in the accuracy in the bump bonding technique is to be an important theme in the art. However, the related art has various problems as mentioned above, and a technique of fully ensuring high producibility and high bonding reliability in bump formation on electrodes of electronic parts is now greatly desired.
With the recent development of portable information terminals and popularization of mobile computing of carrying and using portable computers, electronic appliances are further down-sized more and more. The circuit boards to be built in these electronic appliances are desired to be much more down-sized and thin-sized. In addition, with the popularization of electronic appliances that are required to satisfy high-speed operation, such as communication appliances, desired are circuit boards suitable to high-speed operation that enables accurate switching to high-frequency signals. In such circuit boards, it is desired to shorten the wiring length and to reduce the wiring width and the wiring distance for the purpose of shortening the time necessary for electric signal propagation. To that effect, circuit boards are desired to have an increased wiring density so as to attain high-density packaging thereon, in accordance with the recent tendency toward down-sized and high-speed-operable electronic appliances.
Various production methods have heretofore been proposed for circuit boards. One method recently proposed comprises pressure-bonding an adhesive tape prepared by forming a metal foil circuit pattern on the surface thereof (circuit transfer tape) to an insulating substrate to thereby embed the circuit pattern into the insulating substrate, and thereafter peeing the adhesive tape from the circuit pattern so as to transfer the circuit pattern onto the insulating substrate.
The above-mentioned production method is described with reference to FIG. 10. As in FIG. 10(a), a circuit transfer tape 2101 is prepared. The circuit transfer tape 2101 comprises an adhesive tape 2102 and a metal layer 2103 to form a circuit pattern on its surface. After this is positioned to an insulating substrate 2104, the two are bonded to each other. The insulating substrate 2104 is a semi-cured prepreg. In this case, the two are bonded in a mode of thermal pressure bonding, for example, under a pressure of 100 kg/cm2 or so, and as in FIG. 10(b), a part or all of the metal layer 2103 is embedded and fixed in the insulating substrate 2104.
Next, as in FIG. 10(c), the adhesive tape 2102 is peeled from the insulating substrate 2104, whereby a metal layer 2103 having a circuit pattern is transferred onto the insulating substrate 2104 to form a single-layered circuit board. Further, if desired, the insulating substrate 2104 is completely cured. A multi-layered circuit board may be formed by laminating the above-mentioned single-layered circuit boards.
However, when the metal layer 2103 is transferred onto the insulating substrate 2104, the adhesive component of the adhesive tape 2102 is contacted with the insulating substrate 2104 and the adhesive tape 2102 is often difficult to peel from the insulating substrate 2104. In particular, when a fine circuit pattern is transferred and when the adhesive tape 2102 strongly adheres to the insulating substrate 2104, then the semi-cured insulating substrate 2104 may be deformed while the adhesive tape 2102 is peeled off, whereby the wiring distance of the metal layer 2103 may be disordered and the wiring planarity may be lost, or that is, the circuit pattern may be disordered, and, in addition, the embedded metal layer 2103 may be peeled off along with the adhesive tape 2102.
To solve the problems as above, Patent Reference 5 discloses a method for producing a circuit board, which comprises using a circuit transfer tape with a layer of a photocrosslinkable adhesive component having high adhesiveness formed thereon, and in which the circuit transfer tape is exposed to light from the side of the circuit pattern thereof so as to lower the adhesive force of the tape on the side with no circuit pattern formed thereon, and therefore even when the adhesive component of the circuit transfer tape is contacted with an insulating substrate, it does not adhere to the insulating substrate and the circuit pattern can be thereby well transferred onto the insulating substrate with no pattern disturbance.
However, even when the photocrosslinkable adhesive is crosslinked through exposure to light as in Patent Reference 5, the reduction in the adhesive force owing to the light irradiation is limited, and when a microfine and high-density circuit pattern is transferred onto an insulating substrate, then the circuit pattern may be often broken when the transfer tape is peeled off. In addition, when a circuit pattern having an extremely narrow line width is transferred onto an insulating substrate, then the circuit transfer tape must be exposed to light from the side of the adhesive tape so as to lower the adhesive force between the metal foil and the photocrosslinkable adhesive. In this case, therefore, the light exposure step must be carried out many times repeatedly, and this is problematic in that the production efficiency (producibility) and the workability are thereby lowered.
At present, in production of semiconductor devices, a resist pattern is formed through photolithography, and various materials of conductor films, semiconductor films or insulator films for semiconductor devices are patterned through etching via the resist pattern serving as a mask (for example, see Non-Patent Reference 1). In addition, the resist pattern is used as a mask for plating, and a metal film of copper (Cu) or gold (Au) may be patterned. The patterning process through such photolithography is at present the most popular process in patterning not only for production of semiconductor devices but also for production of any other various display devices or micromachines.
Patterning through photolithography is described with reference to FIG. 16 and FIG. 17. FIG. 16 and FIG. 17 are cross-sectional views showing a process scheme of forming a resist pattern through photolithography and forming a pattern on an insulator film by the use of the resist pattern serving as an etching mask.
As in FIG. 6(a), an insulator film 3102 of silicon oxide is formed on a semiconductor substrate 3101 with various semiconductor elements (not shown) formed thereon, and a resist film 3103 is formed on the insulator film 3102 in a coating process of photolithography. In this, the resist film 3103 is formed of a photosensitive resin, and its formation is as follows: Concretely, a polymer of a photosensitive resin composition or the like is dissolved in a solvent to prepare a resist coating solution, and this is applied onto a semiconductor wafer, the insulator film 3102 formed on the semiconductor substrate 3101 in a mode of spin coating. After thus spin-coated, this is prebaked at a temperature not higher than 100° C. so as to vaporize away the unnecessary solvent. In that manner, the resist film 3103 is formed.
Next, as in FIG. 16(b), using a reticle 3106 that comprises a quartz glass substrate 3104 and a masking pattern 3105 formed on its surface, as a photomask for exposure in photolithography, the resist film 3103 is exposed to light 3107 in an ordinary reduction projection exposure manner for pattern exposure transfer to it. In that manner, an optical pattern transfer region 3103a is formed in a predetermined region of the resist film 3103. Next, this is heat-treatment for PEB (post exposure baking). The recent exposure light 3107 is an ArF excimer laser (wavelength: about 193 nm), and the resist film 3103 is a chemical amplification-type resist.
Next, for development in photolithography, the semiconductor substrate 3101 that is in a semiconductor wafer condition is dipped in a developer. If desired, this may be exposed to shower of a developer. In that manner, the optical pattern transfer region 3103a is removed through the development to form a resist pattern, as in FIG. 16(c). Further, this is post-baked at a temperature of 120° C. or so, thereby forming a resist mask 3108 on the insulator film 3102. This is a case where the resist is a positive resist. The other case where the resist is a negative resist is contrary to it, in which the optical pattern transfer region 3103a remains through the development and the other region is removed.
Next, using the resist mask 3108 as an etching mask, the insulator film 3102 is dry-etched by the use of plasma, as in FIG. 17. With that, an opening 3109 is formed in the insulator film 3102.
Next, the resist mask is stripped off. As in FIG. 17(b), the resist mask 3108 is ashed away in oxygen (O2) plasma. In that manner, a pattern of the insulator film 3102 having the opening 3109 is formed. In this, the opening 3109 is generally to be a via-hole for wiring interconnection via the insulator film 3102 that serves as a wiring interlayer insulation film.
Recently, the pattern dimension in patterning semiconductor devices has become significantly micro-sized to a level of 100 nm or less. With the advanced micropatterning technology, semiconductor devices are therefore much more improved to have an increased degree of integration and to be multi-functional high-performance devices.
However, the above-mentioned micropatterning technique in the above-mentioned photolithography technology requires the above-mentioned high-level techniques of coating, exposure, development and stripping, and it increases the production costs in the process of patterning semiconductor devices.
Heretofore, the above-mentioned coating, exposure and development steps for resist mask formation through photolithography constitute the basis of photolithography, and even when the technique for these steps cold be improved individually, the basic constitution of the series of these techniques would not change. The same may apply also to the step of stripping a resist mask as above. Improving these techniques to higher-leveled ones inevitably brings about the increase in the cost for patterning. Accordingly, it is desired to develop a novel technique that basically differs from conventional techniques in that it satisfies both high-level micropatterning technology and reduced production costs.
[Patent Reference 1]JP-A 11-293207[Patent Reference 2]JP-A 2000-355618[Patent Reference 3]JP-A 11-217458[Patent Reference 4]JP-A 11-168123[Patent Reference 5]JP-A 10-178255[Non-Patent Reference 1] Semiconductor Handbook, 2nd Ed., edited by Semiconductor Handbook Edition Association, published by Ohm, May, 25, 1994, pp. 198-199.
In consideration of the current situation as above, an object of the present invention is to provide a heat-decaying material which hardly deteriorates or decomposes at ordinary service temperatures but which may decay within a short period of time when heated at relatively low temperatures.
Another object of the invention is to solve the above-mentioned drawbacks in the related art and to provide a method for producing a porous film in which sufficiently large pores may be formed uniformly and the material to form the holes do not remain, and also to provide a porous film obtained according to the production method.
Further, the invention has been made in consideration of the current situation as above, and its object is to provide a transfer sheet that has the following advantages: Using it, conductive fine particles may be disposed on the electrodes of electronic parts not being shifted or dropped from it; or multiple conductive fine particles may be surely and readily melt-bonded to the electrodes of electronic parts, all at a time thereon. Even when the line width of circuit patterns is thinned, the circuit pattern may be transferred onto a semi-cured insulating substrate or a ceramic green sheet, not being disordered. Accordingly, using the transfer sheet makes it possible to obtain high-density and high-accuracy micropatterned wiring circuit boards.
Still another object of the invention is to provide a novel technique for patterning so as to provide a patterning method that makes it possible to simplify the above-mentioned series of techniques and to reduce the costs of the patterning technique.